JavaScript

What Javascript Spread Operator is, How It Works and How to Use It

What Javascript Spread Operator is, How It Works and How to Use It feature image

JavaScript spread operator is one of the more popular features that were introduced in ES6. This tutorial will help you understand it. You will learn what spread operator is and how it works. You will also learn how to use it to copy and merge arrays and object literals, insert data and more.

Introduction to JavaScript spread operator

Spread operator is a feature that allows you to access content of an iterable object. Iterable object is an object, or data structure, that allows to access its content with for…of loop. The most popular example of an iterable is an array. Another example of an iterable can be objects literals or strings.

When you wanted to access all content in some iterable, before spread operator was a thing, you had to use some kind of a loop, such as the mentioned for...of loop, or method, such as forEach(). Another option were indexes. Spread Operator allows you to do this much faster and with much less code. About the syntax.

The syntax of spread operator is simple and easy to remember. It consists of three dots (...). These three dots are followed by the iterable (...someIterable), whose content you want to access.

// Create array
const myArray = ['Venus', 'Ares', 'Mars']

// Use spread operator to access content of "myArray" variable
// Syntax: ...someIterable
console.log(...myArray)
// Output:
// 'Venus' 'Ares' 'Mars'

Spread operator and object literals

When you want to use spread operator with object literals the syntax is the same. You will use those three dots, but now followed by the name of the object whose content you want to access. The result you will get will be the content, only without the surrounding curly braces.

// Create object literal
const myObj = {
  firstName: 'Sam',
  lastName: 'Dodge'
}

// Use spread operator to access content of "myObj" variable
// Note: the {} around ...myObj are to avoid TypeError
// console.log(...myObj) would not work
console.log({ ...myObj })
// Output:
// { firstName: 'Sam', lastName: 'Dodge' }

Duplicating iterables with spread operator

The spread operator allows to quickly access content of an iterable. This can be useful when you want to copy iterable objects. You may not know this, but copying objects can be tricky. When you try to copy some primitive, like a number or a string, you will create a real copy, or clone. This is called deep copy.

Deep and shallow copies

This is not true for objects, including iterables. When you try to copy an object, such as array, you will not create a real copy. What will happen instead is that JavaScript will create new reference for that object. You can think about this as creating new alias or name.

When you copy an object, you only create new alias for it. As a result, you have two names for that thing, that object. However, there is still only one object, not two. This also means that if you do something with the object using the second alias (reference), those changes will also have effect on the first alias.

Remember, there is still only one objects, but two references (aliases) to access it. This type of copy is called shallow copy and this type of copying is called copying by reference.

// Create an array
const myArray = ['Spread', 'Rest', 'JavaScript']

// Create shallow copy of "myArray" variable
const myShallowCopy = myArray

// Log the content of "myArray"
console.log(myArray)
// Output:
// [ 'Spread', 'Rest', 'JavaScript' ]

// Log the content of "myShallowCopy"
console.log(myShallowCopy)
// Output:
// [ 'Spread', 'Rest', 'JavaScript' ]

// Remove the last item from the original array ("myArray")
myArray.pop()

// Log the content of "myArray" again
// The last item is gone as it should
console.log(myArray)
// Output:
// [ 'Spread', 'Rest' ]

// Log the content of "myShallowCopy" again
// The change of "myArray" will also appear in "myShallowCopy"
// The last item is also gone
console.log(myShallowCopy)
// Output:
// [ 'Spread', 'Rest' ]

Deep copies with spread operator

This is how copying in JavaScript works automatically. The good news is that spread operator allows you to overcome this issue with shallow copies. It allows you to quickly create deep copies of iterables. So, no more worries about changes happening in unexpected places.

Creating a real, deep, copy of some iterable with spread operator is simple. First, create a variable and assign it some iterable, some array. This will be the iterable you will copy. Second, create new variable. To assign this new variable a copy of the first you will use the spread operator followed by the name of first variable, wrapped with square brackets.

The spread operator will access the content and basically strip away the square brackets of the original array. So, in order to re-create the array, you will wrap the content in new pair of square brackets. That’s it. You have a new deep copy of the first iterable, in this case the original array.

If you decide to change the original array, or the copy, that change will have effect only for that specific array.

// Create the original array
const myArray = ['Spread', 'Rest', 'JavaScript']

// Use spread operator to create deep copy of "myArray""
const myDeepCopy = [...myArray]

// Log the content of "myArray"
console.log(myArray)
// Output:
// [ 'Spread', 'Rest', 'JavaScript' ]

// Log the content of "myDeepCopy"
console.log(myDeepCopy)
// Output:
// [ 'Spread', 'Rest', 'JavaScript' ]

// Remove the last item from the original array "myArray"
myArray.pop()

// Log the content of "myArray" again
// The last item is gone as it should
console.log(myArray)
// Output:
// [ 'Spread', 'Rest' ]

// Log the content of "myDeepCopy" again
// The "myDeepCopy" is not affected by change made to "myArray"
// The last item is still there as it should
console.log(myDeepCopy)
// Output:
// [ 'Spread', 'Rest', 'JavaScript' ]

Deep copies of object literals with spread operator

Just like you can create deep copies of arrays you can also create deep copies of object literals. The syntax is almost the same. You have to use those three dots followed by the name of the object literal you want to copy. You will then assign this into a new variable. Just make sure to wrap this whole thing in curly brackets, not square.

// Create the original array
const myObj = {
  title: 'Guards! Guards!',
  author: 'Terry Pratchett',
}

// Use spread operator to create deep copy of "myObj""
const myDeepCopy = { ...myObj }

// Log the content of "myObj"
console.log(myObj)
// Output:
// { title: 'Guards! Guards!', author: 'Terry Pratchett' }

// Log the content of "myDeepCopy"
console.log(myDeepCopy)
// Output:
// { title: 'Guards! Guards!', author: 'Terry Pratchett' }

// Add new property the original object "myObj"
myObj.format = 'Hardcover'

// Log the content of "myObj" again
// New property is there
console.log(myObj)
// Output:
// {
//   title: 'Guards! Guards!',
//   author: 'Terry Pratchett',
//   format: 'Hardcover'
// }

// Log the content of "myDeepCopy" again
// The "myDeepCopy" still contains only two properties
console.log(myDeepCopy)
// Output:
// { title: 'Guards! Guards!', author: 'Terry Pratchett' }

Note: Creating deep copies with spread operator will work only for first-level items. It will not work for nested arrays ad objects. For this, you can use const myClone = JSON.parse(JSON.stringify(objToClone)).

Merging with spread operator

Another thing you can do with spread operator is merging two or more iterables. Previously, when you wanted to merge two or more arrays for example you would have to use some method such as concat(). Spread operator allows you to do this just as fast. If not faster. With easier syntax.

The process is similar to copying existing array. You create new array. Next, you use the spread operator along with the names of first array you want to merge. This array will be followed by comma and another spread followed by name of the second array. Finally, you will also wrap this inside a pair square brackets.

The result you will get will be all items from the arrays you wanted to merge inside a single array.

// Create first array
const arrayOne = [1, 2, 3]

// Create second array
const arrayTwo = ['four', 'five', 'six']

// Merge first and second array using spread operator
// Syntax: [...arrayOne, ...arrayTwo, ...arrayThree, etc.]
const arrayMerged = [...arrayOne, ...arrayTwo]

// Log the content of "arrayMerged"
console.log(arrayMerged)
// Output:
// [ 1, 2, 3, 'four', 'five', 'six' ]

Merging object literals with spread operator

Merging object literals with spread operator works just like with arrays. The only difference in syntax is that you have to use curly brackets instead of square brackets to wrap everything. The rest is the same and the result is a new object literal with merged content of the object literals you specified.

// Create first object
const myObjOne = {
  title: 'The Color of Magic',
  author: 'Terry Pratchett',
}

// Create second object
const myObjTwo = {
  publicationDate: '2009',
  format: 'Paperback'
}

// Create third object
const myObjThree = {
  language: 'English',
  genre: 'Fantasy'
}

// Use spread operator to merge "myObjOne", "myObjTwo" and "myObjThree"
const myMergedObj = { ...myObjOne, ...myObjTwo, ...myObjThree }

// Log the content of "myMergedObj"
console.log(myMergedObj)
// Output:
// {
//   title: 'The Color of Magic',
//   author: 'Terry Pratchett',
//   publicationDate: '2009',
//   format: 'Paperback',
//   language: 'English',
//   genre: 'Fantasy'
// }

Inserting data with spread operator

We discussed how to use spread operator to access content of arrays and object literals. We also discussed how to use it to duplicate those iterables and even merge them. This is not all you can do. You can also use spread operator to data. You can take content of one iterable and insert it inside another iterable.

For example, let’s say you have two arrays with some content. Spread operator allows you to insert the content of one anywhere inside the second. You can do this also with object literals, insert content from one anywhere inside another. Or, you can insert object literal into an array or vice versa.

// Example no.1: Arrays
// Create first array
const myArrayOne = ['C', 'C++', 'Java']

// Create second array with some content
// plus all the content from "myArrayOne"
const myArrayTwo = ['JavaScript', 'Python', 'PHP', ...myArrayOne, 'Assembly']

// Log the content of "myArrayTwo"
console.log(myArrayTwo)
// Output:
// [ 'JavaScript', 'Python', 'PHP', 'C', 'C++', 'Java', 'Assembly' ]


// Example no.2: Object literals
// Create first object literal
const myObjOne = {
  numOfPages: 706,
  publisher: 'O\'Reilly Media'
}

// Create second object literal with some content
// plus all the content from "myObjOne"
const myObjTwo = {
  title: 'JavaScript: The Definitive Guide',
  author: 'David Flanagan',
  ... myObjOne, // insert the content of "myObjOne"
  language: 'English'
}

// Log the content of "myObjTwo"
console.log(myObjTwo)
// Output:
// {
//   title: 'JavaScript: The Definitive Guide',
//   author: 'David Flanagan',
//   numOfPages: 706,
//   publisher: "O'Reilly Media",
//   language: 'English'
// }

Spread operator and function arguments

When you use spread operator to access content of an iterable, it will return only the content. It will remove the surrounding square brackets in case of an array or curly brackets in case of an object literal. This can be handy when you want to call a function that takes some arguments.

Instead of passing each argument one by one, you can pass in an array with all arguments preceded by spread operator. The result will be the same as if you would pass all arguments one by one.

// Create an array with something
// that will be used as arguments
const myArgs = ['Jack', 'Joe', 'Tony']

// Create a simple function
// that will return all arguments one by one
function sayNames(name1, name2, name3) {
  return `Names you passed are ${name1}, ${name2} and ${name3}.`
}

// Call sayNames() function using spread operator
// to pass in content of "myArgs" as arguments
sayNames(...myArgs)
// Output:
// 'Names you passed are Jack, Joe and Tony.'

You can also use spread operator with math functions, that accept multiple values as arguments, to pass in those values.

// Create an array with numbers
const numbers = [15, 3, -5, 84, 653, Infinity]

// Get the highest number inside "numbers"
const highestNumber = Math.max(...numbers)

// Get the lowest number inside "numbers"
const lowestNumber = Math.min(...numbers)

// Log the value of "highestNumber"
console.log(highestNumber)
// Output:
// Infinity

// Log the value of "lowestNumber"
console.log(lowestNumber)
// Output:
// -5

Using spread operator with strings

In the beginning of this tutorial we discussed that spread operator can be used with iterable objects. Well, only with them. I mentioned that one of these iterables are also strings. This may sound weird, but it is true. JavaScript allows you to use spread operator also with strings.

When you use spread on a string, the result will be the same as if you would use split() method. In other words, spread operator will convert the string into an array. You will get that string in the form of individual characters, that is words, digits and whitespace.

// Create some text
const text = 'Sunny day.'

// Use spread to convert the string into an array
const processedText = [...text]

// Log the content
console.log(processedText)
// Output:
// [ 'S', 'u', 'n', 'n', 'y', ' ', 'd', 'a', 'y', '.' ]

Conclusion: What JavaScript spread operator is, how it works and how to use it

JavaScript spread operator is very easy to use. It allows you to do a lot of things with only small amount of code. I hope this tutorial helped you understand what JavaScript spread operator is, how it works and how to use it.

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